An aluminium or aluminium alloy is often used in the field unsuitable for steel because it is light and easy to process and cast, as well as because of its property such as light, heat, electrical or vacuum property. In addition, the technological progress makes it to be easily joined, so it is utilized not only in the filed of building and transportation but also in the various fields such as automobile, electric car, airplane and consumer electronics. In recent years, furthermore, the amount of recycled aluminium is also large for various reasons such as environmental issue and being easy to recycle aluminium, then the demand is tending to increase.
In these circumstances, for aluminium and aluminium alloy, a surface treatment such as hexavalent chromate treatment for the purpose of improvement in its corrosion resistance and the adhesion with paints is applied. However, said hexavalent chromate uses hexavalent chromium as main component, so hexavalent chromium is contained not only in the treatment solution but also in the hexavalent chromium coating. This hexavalent chromium is pointed out to have a bad influence upon human body and environment, so that the motion to restrain the use of hexavalent chromium has become brisk recently.
Although the surface treatment method free from hexavalent chromium using trivalent chromium has been disclosed as one of the alternate techniques (for example, JP-A-6-173027, JP-A-7-126859, JP-A-11-152588 and JP-A-11-335865) (the term “JP-A” as used herein means an “unexamined published Japanese patent application”), such a method has not adequate corrosion resistance and adhesion with paints equivalent to those of hexavalent chromium and thus, under the present situation, said method is not industrialized.